1. Field of the Invention
The present invention relates to a dielectric resin composition, and more specifically it relates to a dielectric resin composition comprising an epoxy resin and a cyanate ester which would react with the epoxy resin, and to a dielectric resin composition comprising a polyimide with epoxy groups on the polymer side chain and a cyanate ester which would react therewith. Cured resins obtained from the composition of the invention have low dielectric constant, and the compositions are therefore particularly suitable for use as interlayer dielectric in multilayer circuit boards such as MCM-L/D (Multi-Chip Module-Laminate/Deposited) boards or single chip packages, such as CSP.
2. Description of the Related Art
Printed circuit boards are commonly used to integrate electronic parts into electronic devices. The usual process for manufacturing printed circuit boards involves hot pressing together a dielectric resin layer and a copper or other metal layer as the wiring layer, followed by etching of the metal layer according to an electronic circuit pattern. This process is cost effective, but cannot be applied to manufacture high-density circuit boards. On the other hand, build-up multilayer wiring boards have long been used for hybrid ICs, and these are fabricated by alternately printing a patterned conductor and a prepreg of a dielectric on a ceramic substrate in stacked layers and baking them together.
Due to increasing demand for small feature sizes, higher performance and low cost for electronic devices in recent years, high-density wiring boards have become essential. In the field of printed boards, the rapidly increasing miniaturization and multilayer constructions of electronic circuits and higher density mounting of electronic parts has led to more active research on thin-film multilayer build-up printed circuit boards. In build-up printed circuit boards, the dielectric layers and patterned copper wiring layers are alternately stacked, and interconnections between upper and lower wiring layers are established through via holes formed in the dielectric layers.
A common method for manufacture of build-up printed boards will now be explained, with reference to FIG. 1. First, both sides of the core substrate 1 which comprises a dielectric resin reinforced with a filler such as glass fibers, copper wiring layers 7 with fine patterns on both sides thereof, throughholes 5 (filled with a dielectric resin 4) to connect the wiring layers 7 on both sides, and a wiring layer 8 formed inside the substrate itself are coated with a photosensitive dielectric material, and are then exposed to lights (or irradiated) and developed to form the dielectric layer 2 with via holes. A copper thin film is then deposited on the dielectric layer 2 by electroless plating followed by electroplating, and this Cu layer is patterned to form the copper wiring layer 3. Next, if necessary, the steps from formation of the dielectric layer 2 to formation of the wiring layer 3 are repeated to produce a multilayer circuit board 10. Thereafter, the solder resist layer 22 is deposited, irradiated and developed. Finally, solder bumps 21 are formed on the uppermost wiring layer of the circuit board in order to make connection with the outer circuit.
The photosensitive resin composition used to form the dielectric layer of the multilayer circuit board can be developed either by an organic solvent or an aqueous alkaline solution. Examples of photosensitive resin compositions that can be developed by alkaline agents include photosensitive compositions in which base polymers are reaction products of unsaturated monocarboxylic acids with epoxy resins, with further addition of polybasic acid anhydrides (Japanese Examined Patent Publication No. 56-40329 (JP-B-56-40329) and Japanese Examined Patent Publication No. 57-45785 (JP-B-57-45785)), and alkali-developing photosensitive compositions with excellent heat resistance and chemical resistance, which employ novolac-type epoxy resins (Japanese Unexamined Patent Publication No. 61-243869 (JP-A-61-243869)).
In order to obtain a high-density build-up printed circuit board it is necessary to create micro-vias in the build-up dielectric layers. Since micro-vias are usually created by laser methods, the dielectric materials must be laser compatible, or in other words, the dielectric material should not contain materials such as glass fiber reinforcements. The dielectric material should be ductile enough to get a build-up printed board with no micro-cracks. Thus, unlike the core substrate, the material for the build-up dielectric layer must be selected with great care.
Epoxy resins cured with amines, acid anhydrides or phenols are widely used as dielectric materials in printed circuit boards mainly because of their low cost and ease of processing. However, since epoxy dielectric materials are thermally unstable at temperatures exceeding 200xc2x0 C., and especially in temperature ranges exceeding 250xc2x0 C., and have high dielectric constant and dielectric loss (due to free xe2x80x94OH and xe2x80x94NH groups in the cured product), and high flammability, they are found to be not promising materials in high-density printed circuit boards. Although epoxy materials with high percentage of bromine are currently in wide use to improve the flame resistance of dielectric materials, it has been found that if brominated dielectrics are used in fine-patterned printed circuit boards, Br migration occurs at high temperature and high humidity conditions.
Epoxy/cyanate ester resins with no xe2x80x94OH or xe2x80x94NH groups have been reported in Electronic Materials Handbook, Volume 1, Packaging (1989), p.606, and these may be expected to exhibit satisfactory thermal characteristics and electrical characteristics compared with the conventional epoxy resins cured with amines, acid anhydrides or phenol curing agents. However, these resins are very brittle, and their thermal and electrical characteristics are largely dependent on the epoxy/cyanate ester ratio, the curing conditions and the curing catalyst. Although some limited studies have been reported on epoxy/cyanate ester systems, no reports have been found regarding materials suitable for high-density printed boards.
The use of polyimide resins is known to be effective to eliminate the problems described above, but polyimides have to be cured at high temperatures ( greater than 350xc2x0 C.) and therefore require to use with highly heat-resistant core substrates. Because of the high cost of highly heat-resistant substrates, particularly silicon or aluminum nitride substrates, it is very difficult to produce low cost PWBs with polyimide dielectric materials. Other low temperature processable dielectric materials include soluble polyimides, but since the cured films of the polyimides are susceptible to solvents, it is difficult to use them in multilayer circuit boards. Although numerous low-temperature curing imide oligomers with thermosetting reactive groups on both ends have been reported, due to their low molecular weights, it is very difficult to get defect-free dielectric film on substrates.
An object of the present invention is to provide a dielectric resin composition with promising thermal and electrical characteristics, and particularly dielectric characteristics, which can form cured films with excellent mechanical properties and moisture resistance, and which is particularly suitable for formation of interlayer dielectrics of multilayer circuit boards such as MCM-L/D boards or single chip packages. Another object of the invention is to provide multilayer circuit boards using this resin composition as the interlayer dielectric.
According to the first aspect, the dielectric resin composition of the invention comprises at least one type of epoxy resin and at least one type of cyanate ester which would react with the epoxy resin, together with a metal ion catalyst. The ratio of the epoxy functional groups of the epoxy resin to the cyanate groups of the cyanate ester is preferably in the range of from 1:0.8 to 1:1.4.
According to the second aspect, the dielectric resin composition of the invention comprises a polyimide resin with side chain epoxy groups, a cyanate ester with two or more cyanate groups in the molecule, and a metal ion catalyst.
Multilayer circuit boards according to the invention comprise a core substrate, and a prescribed number of dielectric layers and wiring layers which are stacked alternately, wherein at least one of the dielectric layers is formed from the dielectric resin composition of the invention.